Temperature measurement and control



April 6 v 1940-" H. R. EGGERS ,197,635

TEMPERATURE MEASUREMENT AND CONTROL Filed Nov. 9, 1937 Thermocouple voltage, E.

Inventor: Hermann R. Eggers,

by JMZ Hi Attorneg.

Patented Apr. 16, 1940' 2,197,635

UNITE-D STATES PATENT OFFlCE TEMPERATURE MEASUREMENT AND CONTROL Hermann It. Eggers, Berlin, Germany, assignor to General Electric Company, a corporation of New York Application November .9, 193'], Serial No. 173,721 In Germany November 16, 1936 2. Claims. (Cl. 73-361) My invention relates to temperature respon- The invention may be understood more readily, sive apparatus and concerns particularly apparafrom the following detailed description when contus utilizing thermocouples for measuring or for sidered in connection with the accompanying automatically controlling temperatures. drawing and those features of the invention It is an object of my invention to provide temwhich are believed to be novel and paten le 5 perature responsive apparatus of the -thermowill be pointed out in the claims appended hereto. couple or thermojunction type having increased n t e dr Fig. 1 s a circuit diagram p accuracy and reliability and which is relatively seating-the potentiometer connection of the prior unafiected by variations in resistance of the for either indicating controlling p thermocouple or its leads or by variations in ture; 2 is a circuit diagram of one embodil0 E. M. F. of an auxiliary voltage source. merit of y invention; Fig. 3 is a graph explain- Another object of my invention i t provide ing the principle of operation of my invention; aconnection for automatic temperature control and ig- 4 s a circuit diagram o another e apparatus in which the heating appliance will be bodiment of my invention. Like reference charl5 t r ofi i a of failure of th thermocouple acters are utilized throughout the drawing to instead of being held on continuously and raiss n e like P ing the temperature to a dangerous level. n e Prior artv arrangement of a st111 another object of my invention i t r thermocouple II is connected in series with an vide cold junction temperature compensation. auxiliary voltage rc n a rr respon- Other and further objects and advantages will sive' device, such as an indic t g o re o di 0 become apparent as the description proceeds. ammeter and a relay The relay C011- There are in general two arrangements of trols a source of heat IS in a furnace or heating thermocouple temperature responsive devices in ys represented y t esistor ll. The conuse; the direct connected arrangement, and the nections are such that the voltage E of the couple potentiometer arrangement. In the former ar- II, as shown by the arrow is in a direction DD 25 rangement, an electric measuring instrument is g t e V ltage source and the magnitude V of connected directly to the thermocouple and the the voltage source i2 is such that at a predeter accuracy of the reading depends upon constancy mined temperature the voltages are balanced and of the resistance of the thermocouple and the no c rr nt flows in the thermocouple II. When connecting leads. The resistance is not always the temperature exceeds apredetermined value 30 known; it depends upon the temperature and the relay I l opens and cuts ofi the source of heat varies with the length of service of the thermo- I5. How ver, in c the thermocouple II should couple. In the potentiometer arrangement an burn out, which may readily happen, the relay auxiliary voltage is connected in opposition to M w l re e e ed and Cl sed regardless that of the thermocouple so that no current flows of he emperature, causin the heater I5 to re- 35 in the measuring instrument at a given temperamain in circuit indefinitely so that dan erous ture and the effect of resistance variations is v rh y result. minimized. However; the reading of the instru- In order to overcome this hazard and in order ment is greatly affected by variations in the magto make the temperature. indication independent nitude of the auxiliary voltage and the same of variation in the voltage-0f the source I2, I 40 measuring instrument cannot be employed if the co nect t e current e p e v s n the auxiliary voltage is removed. cross arm or diagona o a dge Circuit such In carrying out my invention in its preferred as s w in f example The thermoform, an auxiliary voltage of such magnitude as couple ll constitutes One arm Ofthe r dge and to cause no current to flow at a particular temresistors l6, I1 and iii are added to form the 45 perature, which may be any desired temperaremaining arms of a bridge, the arms H and I! ture, is applied in opposition to the thermobeing connected in series to the voltage source l2, couple voltage. However, in order to make the and the arms "3 a d s, se be Connected apparatus independent of the magnitude of the in'series to the source l2. Forthe sake of simauxiliary voltage, a bridge connection is chosen, plicity, only the milliammeter. I3 is shown to 50 in which the indicating or recording apparatus represent whatever type of current responsive lies in the diagonal arm of the bridge and the device is connected in the cross arm, but it will thermocouple lies in a bridge arm.. The resistbe understood that the connection of Fig. 2is not ances are so chosen that the instrument current limited to temperature measurement and may 5 is made independent of the auxiliary voltage. also be used. for temperature control. The resistor I6 is suchas to have a substantially constant resistance 1', which bears a relationship to the resistance R of thermocouple II which will later be explained. The resistances a and b 01' 5 the bridge arms I! and I8 also have a relationship which will later be explained.

The manner of operation of the apparatus will be explained by considering the relationship between the currents in various parts of the bridge.

The following symbols are used:

2 represents the current in the instrument I3 I represents the current in the bridge arm I! I-i represents the current in the thermocouple I I 111 represents the current in the bridge arm 18 n represents the ratio of currents in the. bridge arms I8 and I7.

nI+i represents the current in the resistor l6 g represents the resistance of the instrument 13 R represents the actual resistance of the thermocouple II, including leads r represents the resistance of the resistor l6 E represents the voltage of the thermocouple ll Emax represents the voltage of the thermocouple I I for the maximum measured temperature.

Since the sum of the voltages in any complete circuit must equal zero, one may obtain the following equation by considering-the voltages in the left-hand mesh of the bridge.

5 The following additional symbols are now introduced:

R0 represents the resistance of the thermocouple ll (including leads) assumed in calibration.

A represents the deviation of R from R0 I0 represents the theoretical current flowing in the bridge arm I! for the assumed auxiliary voltage.

A represents the deviation of In from I accord- It will be understood that nr is made equal to R0 either by choice 011 or the choice of the ratio of resistances of arms "and I8.

Fig. 3 consists of a series of graphs representing the relationship between instrument current and thermocouple voltage for various different assumed circumstances. The current i and the i voltage E are plotted as percentages of the values Elsi-responding to maximum measured tempera- If one assumes that the actual value of 'R is that assumed in calibration oi the apparatus, 1. e., that equation Equation 4 becomes in which the current i'is independent of variations A in auxiliary voltage. Equation 5 is plotted in Fig. 3 and is represented by the line 5.

If the auxiliary voltage source l2 were removed so that the current, I, in the bridge arm I! became zero, Equation 2b would become or r J X=1 and Equation 4 would become E 1 g+ 0 o (5a) y+r+R0 If one assumes further that the resistance 01' the thermocouple II is too great and is of such a value that A L g+r+R Equation 5a becomes Equation 6 is plotted as line 6 in Figure 3.

' If the auxiliary voltage is correct but the resistance of the thermocouple is excessive and or the same value assumed for Equation 6, i. e.

\-0 and g+T+R0 0.11

and it one further assumes that maz Equation 7 is plotted inFig. 3 as line I.

If one assumes the same excessive thermocouples resistance and the same theoretical current, Io, as before but assumes that the auxiliary voltage is too small, 1. e.

A=0.1 Equation 4 becomes E 1+0.099-% 8) g+r+R 1.11-

Equation 8 is plotted in Fig. 3 as line 8.

The assumptions made in obtaining the graphs of Fig. 3 may be summarized as follows:

For graph 5 resistance R of the thermocouple is correct, no other assumptions For graph 6 auxiliary voltage removed and resistance R has a predetermined excessive value For graph '7 auxiliary voltage correct. Resistance R as in 6 For graph 8 auxiliary voltage one tenth low. Re-

sistance R as in 6 and 7.

Inasmuch as measurements are usually made in the upper range between about 60 and 90%, it is apparent that the influence of variations of the thermocouple resistance is less with an auxiliary voltage than without and that Ior'a predetermined temperature the eflect of variation in thermocouple resistance can be exactly.com

'pensated. In the curves shown this temperature is the maximum temperature but by proper selection or the auxiliary voltage it can be made any other temperature.

since 11 was the stipulated ratio between the currents. Since 1' was so chosen as to make the bridge would be simply a balanced bridge when the thermocouple resistance R is equal to R0, 1. c. has not varied from the originally assumed value. Mathematically stated,

From Equation 5 it was seen that independently of the value of the auxiliary voltage if the thermocouple resistance remains constant, i. e., does not vary from its assumed value,

E g+r+Ro and for maximum measured temperature In the Equation an it was assumed that the normal aumliary voltage was of such a value that In other words at the maximum measured temperuatre no current would flow in the thermocouple I! even with a change in the resistance of the thermocouple, provided the auxiliary voltage remained constant at the original value; As shown by Fig. 3, line 7, full compensation is obtained at the maximum measured temperature and very good compensation is obtained at other temperatures in the vicinity. Ii full compensation is desired instead at some intermediate temperature, the auxiliary voltage is made such that no current flows in the thermocouple at the desired intermediate temperature at the time of calibration. The curves 5 and I would then intersect at the values corresponding to such an intermediate temperature.

Manifestly in a very precise determination, the

ratio 11 of the currents in the bridge arms I! and If elthera or b is relatively large in comparison with g and, since 1' was chosen to make Without regard to the values of a or b, for the temperature at which no current flows in the thermocouple I l, if its resistance is correct,

=1 and =Qi'l b sigh Whenever conveniently possible, I prefer to have the resistance a and b relatively great in order that the resistance g need not be considered and compensation will be obtained over a relatively wider range of conditions.

When the current in the thermoiunction II is zero I= i and the following equations hold:

Fbr relatively large values of both a and b in comparison with r and R0 If the apparatus of Fig. 2 is used to control temperature and the current responsive device I3 is one to shut oil the heat when a predetermined temperature is reached asexplained in connection with the relay H of Fig. 1, the circuit of Fig. 2 guards against overheating in case the thermocouple II should burn out. In this case, current flows through the current responsive device I3 to shut oh the heat regardless of temperature as a safety measure. Preferably for this purpose the auxiliary voltage of the source I2 is made relatively high since with normal thermo couple resistance the auxiliary voltage has no efiect upon the instrument current but when the thermocouple burns out the auxiliary voltage sends a. large current through the current responsive device l3. If the current responsive device i3 is an indicating instrument, failure of the thermocouple in the foregoing arrangement will be indicated by the pointer passing to its final scale value or beyond.

It was pointed out in connection with Equation 5 and curve 5 of Fig. 3, thatwhen the thermocouple had the resistance assumed, for any temperature whatsoever no change in deflection would occur when the auxiliary voltage was varied. This constitutes a good test for constancy of the resistance of the thermocouple. If the test reveals that the resistance has varied from the assumed value, one or moreof the resistances r, a and b is changed in value until the instrument deflection remains the same, regardless of whether the auxiliary voltage is switched on or oil. I

My invention is also adapted to providing cold junction temperature compensation by means of a temperature responsive electrical resistance. The connections are shown in Fig. 4, which is like Fig. 2, except that I substitute a temperature responsive resistor II for the resistor 18 oi. Fig. 2. It is assumed that the resistances a and b of the bridge arms I! and I8 are large in comparison with the resistances, R and r, of the couple II and the resistor l8, respectively. Ac cordingly, the currents in the bridge arms i1 and i8 are expressed by the respective equations.

' equation where he is the resistance at a reference temperature assumed to be a normal ambient temperature, is the temperature coefllcient of resistance in terms or the same reference temperature, and t: is the temperature of the cold junction of the thermocouple H compared with the assumed normal ambient temperature. It will be understood that the apparatus is so constructed that the cold junction of the couple I l and the resistor l8 rest in proximity and are at the same temperature.

The resistances r, a and b are chosen to satisfy the equation a bu According to Kirchhofl"s law the voltage equation for the left-hand mesh of the bridge in Fig. 4 is from which results, by substitution of the values of L and Ib, in Equations 87' and 8k,

When the temperature to be measured is at a definite 'value at which the thermocouple voltage v a; fl v, (12a) the term fi a drops out of Equation 12 so that the indication is substantially independent of thermocouple resistance. Accordingly, when exact compensation is desired at a particular temperature in the vicinity of which most measurements are expected to fall the values of resistance and auxiliary voltage are chosen to satisfy Equation 12a. For the condition when i=0 and A=0 Equation 11 becomes E+ V az,

W in which the term represents the cold junction voltage or the correction in the net thermocouple voltage E necessary to take into account cold junction temperature is. The thermocouple will have a definite cold' junction voltage E" for a given cold junction temperature is. Accordingly thevalues of resistance and auxiliary voltage are chosen to satisfy the equation:

and the reading is made independent of varia tions in colcljunction temperature.

Inasmuch as the values A, a and t2 will be small quantities the products of AA and mt: can be disregarded in Equation 11 and Equation 13 holds even when variations take place in thermocouple resistance'and auxiliary voltage and therefore the instrument current represents the measured temperature fully compensated for variations in cold junction temperature.

I have herein shown and particularly described certain embodiments of my invention and certain methods of operation embraced therein for the purpose of explaining its principle and showing its application,.but it will be obvious ,to those skilled in the art that many modifications and variations are possible and I aim, therefore, to cover all such modifications and variations as fall within the scope of my invention which is defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A temperature responsive device, of the thermocouple type compensated at a predetermined temperature against variations in resistance of the thermocouple, comprising a bridge circuit including a source of current energizing the bridge, a current responsive instrument in the cross arm of the bridge and a thermocouple in one ofthe main arms of the bridge, the polarity of the voltage of the current source being the opposite of the polarity of the voltage generated in the thermocouple by heat, the magnitude of the voltage of the current source being sufficient to produce zero current in the thermocouple at the temperature for which the apparatus is to be fully compensated.

2. A temperature responsive device of the thermocouple type having cold junction comaieaees pensation comprising a. voltage source having a voltage V, a thermocouple and a resistor connected in series to said voltage source, a resistor having a resistance -r and a temperature varimagnitude of the voltage source being substantially determined by the equation:

Be ar where --e is the cold junction voltage ofthe thermocouple at a given temperature T above the said predetermined temperature, and. oz is the temperature coemcient of resistance of the variable resistance resistor with respect to the same w predetermined tempereture. 

